Micro-fluid ejection devices are classified by a mechanism used to eject fluid. Two of the major types of micro-fluid ejection devices include thermal actuators and piezoelectric actuators. Thermal actuators rely on an ability to heat the fluid to a nucleation temperature wherein a gas bubble is formed that expels the fluid through a nozzle. The life of such thermal actuators is dependent on a number of factors including, but not limited to, dielectric breakdown, corrosion, fatigue, electromigration, contamination, thermal mismatch, electro static discharge, material compatibility, delamination, and humidity, to name a few. A heater resistor used in a micro-fluid ejection device may be exposed to all of these failure mechanisms.
For example, it is well-known that cavitation pressures are powerful enough to pound thru any solid material, from concrete dams to ship propellers. During each fire cycle, the heater resistor may be exposed to similar cavitation impacts. As the gas bubble collapses, a local pressure is generated on the order of 103 to 104 atmospheres. Such cavitation impacts may be focused on a submicron spot of the heater resistor for several nanoseconds. After 107 to 108 cavitation impacts, the heater resistor may fail due to mechanical erosion. Furthermore, because the heater resistor requires extremely high temperatures to ensure homogeneous bubble nucleation, a distortion energy in the heater due to thermal expansion may be generated of the same order of magnitude as the distortion energy imposed by bubble collapse. A combination of thermal expansion and cavitation impacts may lead to premature heater failure.
In order to protect the fragile heater resistor films, the films may be hermetically sealed to prevent humidity driven corrosion, but the surface of the heater resistor is directly exposed to liquid. In the most critical areas of the heater, a minor surface opening due to defect, wear, step coverage, or delamination may lead to catastrophic failure of the heater resistor.
Accordingly, exotic resistor films and multiple protective layers providing a heater stack are used to provide heater resistors robust enough to withstand the cavitation and thermal expansion abuses described above. However, the overall thickness of the heater stack should be minimized because input energy is a linear function of heater stack thickness. In order to provide competitive actuator devices from a power dissipation and production throughput perspective, the heater stack should not be arbitrarily thickened to mitigate the cavitation effects, overcome step coverage issues, overcome delamination problems, reduce electro static discharge, etc. In other words, improved heater resistor reliability by over-design of the thin film resistive and protective layers may produce a noncompetitive product.
Micro-fluid ejection heads may be classified as permanent, semi-permanent or disposable. The protective films used on the heater resistors of disposable micro-fluid ejection heads need only survive until the fluid in the attached fluid cartridges is exhausted. Installation of a fluid cartridge carries with it the installation of a new micro-fluid ejection head. A more difficult problem of heater resistor life is presented for permanent or semi-permanent micro-fluid ejection heads. There is a need, therefore, for a method and apparatus for improving heater resistor life without sacrificing jetting metrics and power consumption.
With regard to the above, exemplary embodiments of the disclosure provide micro-fluid ejection heads having extended life and relatively low energy consumption and methods of making a micro-fluid ejection heads with extended life and relatively low energy consumption. One such micro-fluid ejection head includes a substrate having a plurality of thermal ejection actuators disposed thereon. Each of the thermal ejection actuators includes a resistive layer and a protective layer for protecting a surface of the resistive layer. The resistive layer and the protective layer together define an actuator stack thickness. A flow feature member is adjacent (e.g., attached to) the substrate and defines a fluid feed channel, a fluid chamber associated with at least one of the thermal ejection actuators and in flow communication with the fluid feed channel, and a nozzle. The nozzle is offset to a side of the fluid chamber opposite the fluid feed channel. A polymeric layer having a degradation temperature of less than about 400° C. overlaps a portion of the at least one thermal ejection actuator, and positioned less than about five microns from at least an edge of the at least one actuator opposite the fluid feed channel.
In another embodiment there is provided a method for extending a life of a thermal ejection actuator for a micro-fluid ejection head. A substrate has a plurality of thermal ejection actuators and a protective layer therefor deposited thereon, and has a flow feature member defining a fluid feed channel, a fluid chamber associated with at least one of the thermal ejection actuators and in flow communication with the fluid feed channel, and a nozzle. The nozzle is offset to a side of the fluid chamber distal from the fluid feed channel. The method comprises depositing a polymeric layer having a degradation temperature of less than about 400° C. in overlapping relationship with at least a portion of the at least one thermal ejection actuator. The polymeric layer overlaps less than about five microns of the at least one actuator adjacent an edge thereof distal from the fluid feed channel.
An advantage of at least some of the exemplary embodiments of the disclosure is that heater energy is not increased while the life of the actuators is substantially enhanced. Another potential advantage of at least some of the disclosed embodiments is an ability to vary the life of an ejection actuator without significantly changing the energy requirements for ejecting fluids.